1. Field of Invention
This invention pertains to the art of methods and apparatuses for use with high pressure fluid systems, and more specifically to methods and apparatuses for use in releasing high pressure fluid from an associated pressure vessel.
2. Description of the Related Art
It is well known in various technologies to require the rapid release of pressurized fluid from a pressure vessel in a controlled manner. A prime example is airbags and similar vehicular inflatable restraints which need to release the pressurized fluid quickly and yet also have a requirement that at the very beginning of the discharge the release should be somewhat gentle. Other technologies may also have similar needs. For example safety relief valves for overpressure protection of a pressure vessel may have downstream piping which could be subject to damage from fluid hammer phenomena if the initial opening were instantaneous.
A release mechanism of the type being described typically involves some form of rupture disc. Rupture discs are widely used for overpressure relief and are simple and, if carefully made, predictable. Most commonly they are used in the mode where they spontaneously rupture if the pressure difference across them is too large. In this sense, many pressure vessels in use today in many industries are protected by rupture discs. For use in a situation where the discharge is to be performed selectively, it is possible to use a classical rupture disc by puncturing it with an actuating mechanism which generates mechanical force and motion. However, attention must be paid to insure that a localized puncture will indeed cause the disc to rupture wide open. In practice it is found that if a rupture disc is punctured in a localized place, tearing wide open subsequent to the puncture only occurs if the pressure behind the disc is more than approximately 60% to 70% of the spontaneous rupture pressure. If the pressure is less, then a puncture will remain a localized hole and will not propagate to complete failure of the rupture disc. This is true even for the so-called scored rupture discs, in which indentations are stamped or coined into one side of the disc to promote failure. Thus, such a release mechanism is limited to conditions where the internal pressure remains in the appropriate range, which may not be the case if the pressure vessel is subject to a wide range of temperatures.
In situations where the conditions for the rupture disc to tear open completely upon puncture are not met, a solution may be to simply use enough explosive so that the entire rupture disc is literally blasted away or to drive a sufficiently powerful cutter to cut out the rupture disc all around its circumference. However, this would require a relatively large quantity of explosive and in some circumstances such a quantity of explosive may not be desirable.
Accordingly, one general category of alternative release mechanism is that of a very weak rupture disc supported by a more robust support, wherein the support is withdrawn selectively. One existing example of this is Okada U.S. Pat. No. 4,289,327 in which the support is supported by a link mechanism. In this patent the link mechanism is directly actuated by the motion of a mass in the sensing device which operates on mechanical principles. This is possibly delicate or overly sensitive in that the link is only slightly over-center and is designed so that it can be pushed to the other side of center by a very slight force. Furthermore, electronics are now frequently involved in the process of deciding whether to deploy the inflator, and so it is desirable to actuate the inflator using an electrical signal.
All of the known prior art in the withdrawn-support category of release mechanisms provide a release which is essentially instantaneous producing a very abrupt jump in the flow rate as soon as the rupture disc ruptures. The release of pressurized fluid is such that the flow rate is zero before the event, then rises essentially instantaneously to its maximum value at the beginning of discharge (when the pressure inside the storage vessel is greatest) and then decays. The exiting flow rate as a function of time may approximate a decaying exponential curve. However, in the airbag industry it is desirable to provide a controlled release which is referred to as pulse-shaping. It is desirable that at the very earliest part of the deployment the flow rate should be limited so as to avoid over stressing the bag or attachments during unfolding, and so as to avoid causing injury to an out-of-position occupant who may be positioned too close to the dashboard of the vehicle. After the brief initial period representing unfolding, a faster discharge rate is desired, and the discharge rate eventually decays due to depletion of the source. Even further, in the airbag industry it is desirable that the characteristics of the discharge, especially its earliest portion, be adjustable as a function of vehicular and occupant variables that may be measured just prior to initiation of the discharge. Two other features which are desirable in the airbag industry are, a self-relieving function for overpressure protection, and measuring or monitoring of the pressure of the contents of the vessel.